There are two typical methods for LED lighting control. One of them is constant average current modulation, such as the controller chip RT9271 (http://www.richtek.com/www/Docs/DS9271-13.pdf) of Richtek Technology Corporation, and the other is constant peak current modulation, such as the controller chip AMC7150 (http://www.addmtek.com/Datasheet/DD034-AMC7150%20_A.pdf) of ADDtek Corporation and the controller chip HV9910 (http://www.supertex.com/pdf/datasheets/HV9910.pdf) of Supertex Incorporation.
FIG. 1 is an application circuit given in the data sheet of RT9271. In a controller chip 10, a power switch (not shown) is connected between a switch pin LX and a ground pin GND for being switched in response to a pulse width modulation (PWM) signal to regulate the current ILED supplied to light emitting diodes LED1-LED3. This control scheme requires a rectifier diode D and an output capacitor Cout for filtering so as to provide a stable average current for the light emitting diodes LED1-LED3. However, to reduce the ripple of the current ILED, the output capacitor Cout requires a greater capacitance and thus this prior art requires a larger and more costly component for the capacitor. Further, since the light emitting diodes LED1-LED3 are serially connected on a current path, the output capacitor Cout will need to sustain a high voltage, which also causes this prior art to require a larger and expensive component.
FIG. 2 is an application circuit given in the data sheet of AMC7150. In a controller chip 12, a power switch (not shown) is connected between a current sense pin Is and an output pin OUT for being switched in response to a modulation signal so as to regulate the current ILED supplied to light emitting diodes LED1 and LED2. FIG. 3 is an application circuit given in the data sheet of HV9910. An output pin Gate of a controller chip 14 provides a modulation signal to switch a power switch SW, so as to regulate the current ILED supplied to light emitting diodes LED1 and LED2. The output capacitor Cout in the circuit of FIG. 3 may be removed without destroying the control. As shown in FIG. 4, each of the LED lighting controllers of FIGS. 2 and 3 employs a peak current control scheme which sets a threshold IPK as the peak current. When the current LED rises up to the threshold IPK, a control signal is triggered to decrease the current ILED. For example, in the control scheme of FIG. 3, during a time period Ton where the power switch SW is on, the current ILED increases, as shown by the time period Ton of the waveform 16 in FIG. 4. When the current ILED reaches the threshold IPK, the power switch SW is turned off and the current ILED begins decreasing, as shown in the time period Toff of FIG. 4.
Although the control circuits of FIGS. 2 and 3 can provide stable LED current without the need of large output capacitor Cout as required by the control circuit of FIG. 1, they are inherently disadvantageous to some applications due to the constant peak current control. In such control scheme, when the input voltage Vin varies, the slope and valley value of the LED current vary accordingly and thus it causes variation of the average LED current. The Vin dependent average LED current variation is especially worse in Boost mode or Buck-Boost mode configurations. FIG. 5 depicts a boost LED driver and FIG. 6 shows the variation of the inductor current IL of this driver as a consequence of the variation of the input voltage Vin. During the time period that the power switch SW is on, the inductor current IL flows from the power input Vin toward the ground terminal GND through the power switch SW. Then, after the power switch SW is turned off, the inductor current IL redirects to the light emitting diodes LED1 and LED2. In other words, only when the power switch SW is off, there is current supplied to the light emitting diodes LED1 and LED2. However, when the input voltage Vin is higher, the inductor current IL increases at a higher speed, and thus the power switch SW will have a shorter on time Ton1 for the current threshold IPK to be reached, as shown by the waveform 18 in FIG. 6. On the contrary, when the input voltage Vin is lower, the inductor current IL increases slower, and longer on time Ton2 is required for the power switch SW, so as for the peak current IPK to be reached, as shown by the waveform 20 in FIG. 6. Since the on-time Ton of the power switch SW varies along with the variation of the input voltage Vin, the average current flowing through the light emitting diodes LED1 and LED2 also varies along with the variation of the input voltage Vin, resulting in variation in illumination of the light emitting diodes LED1 and LED2.